1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus using the optical scanner, for example, a digital copier, a laser printer, a laser plotter and a laser facsimile.
2. Description of the Related Art
In a general optical scanner which has been widely known relating to a laser printer and the like, a light beam is deflected by a light deflector, the deflected light beam is focused on a surface to be scanned (hereinafter, referred to as scanned surface) as minute spot light, and the scanned surface is scanned at a constant speed in the main-scanning direction by this spot light. Namely, in such an optical scanner, laser light emitted from a laser light source, for example, is deflected and reflected by the light deflector, the scanned surface such as an image carrier is scanned by this laser light, and the intensity of the laser light is modulated (for example, on, off) in accordance with an image signal, so that an image is formed on the scanned surface.
Recently, with the increase in speed, the development in resolution, and the increase in a writing width of an image forming apparatus, the number of scanning beams for use in an optical scanning system is more likely to be increased, and a color image forming apparatus having a plurality of optical scanners using a plurality of beams has been generally used. As a light source for the above-described beams, for example, a semiconductor laser array in which light-emitting points are one-dimensionally arranged at predetermined intervals and a surface-emitting laser array in which light-emitting points are two-dimensionally arranged at predetermined intervals are used. In such semiconductor laser arrays, a semiconductor laser array in which the number of beams is increased by synthesizing beams from a plurality of semiconductor laser array light sources can be considered as a light source in which light emitting points are two-dimensionally arranged similar to the surface-emitting laser array.
If the light source which emits a plurality of light beams is used for the optical scanner, it is difficult to adjust the intervals of the scanning lines to be formed on the scanned surface by adjusting the rotation of the light source. In order to address this problem, an anamorphic optical element group is provided in an optical system of the optical scanner, and an adjuster, which adjusts the magnification of the optical system in the sub-scanning direction by adjusting the position of the anamorphic optical element in the optical axis direction, is also provided (refer to, for example, JP H04-101112A, JP H08-15625A, JP 2008-76712A).
On the other hand, as described above, with the increase in a writing width of an image forming apparatus, the size of the scanning lens constituting the optical scanner is increased. For this reason, it is necessary to substitute a glass lens with a resin lens in order to reduce costs. Especially in a tandem writing unit, since the number of optical elements is large, the effect of the cost-cutting becomes more prominent.
However, the resin lens has a problem in that the curvature and the refractive index of the lens face are changed by expansion and contraction with the changes in temperature, and the lens performance, especially the focal position of the light spot on the scanned surface is changed. The change in this focal position increases the diameter of the light spot on the scanned surface, and deteriorates the resolution of the optical scanning. Moreover, in the multi-beam optical scanner, the intervals of the scanning lines to be formed on the scanned surface are changed, so that the quality of the image is deteriorated.
The changes in the focal position and the intervals of the scanning lines by the expansion and contraction of the resin lens with the changes in temperature are generated in an opposite manner between a positive lens and a negative lens. In order to correct these changes, a technique is known, which arranges a pair of resin lenses each having an opposite power on an optical path from a light source to a light deflector, and cancels the changes in the focal position with the temperature changes of the scanning lens (refer to, for example, JP H08-160330A, JP H08-292388A, JP 3483129B).
JP H08-160330A describes an optical scanner including a light source, an incident optical system, a deflector, a scanning optical system and a scanned medium. The incident optical system includes a first optical system (collimated lens) which changes a diverging light beam from the light source into a parallel light beam, and a second optical system which focuses the light from the light source via the first optical system near the deflector in the sub-scanning direction. The first optical system or the second optical system includes a resin optical element (lens) having a negative power in the sub-scanning direction. JP H08-292388A describes an optical scanner including a first focusing unit which focuses near a deflected position of a deflector and has a resin negative lens having a negative refractive power only in the sub-scanning direction, so as to compensate the temperature of the focused position. JP 3483129B describes an optical scanner including an optical system which self-corrects the focal position gap of the light spot on a scanned surface with the changes in environmental temperature by providing between a coupling optical system and a light deflector at least a pair of a resin lens including an anamorphic face having a negative power in both of the sub-scanning direction and the main-scanning direction and a glass lens including an anamorphic face having a positive power at least in the sub-scanning lens.
In order to prevent the deterioration in image quality by the expansion and contraction of the resin lens with the changes in temperature, it is considered to provide a temperature correction function by arranging a resin lens having a negative power and a function which adjusts the magnification in the sub-scanning direction by arranging a plurality of anamorphic optical elements in an optical system before a light deflector in a multi-beam optical scanner using a two-dimensionally arranged light source. However, if a plurality of anamorphic elements is used in the optical system, the focused spot diameter on the scanned surface and the intervals of the scanning lines to be formed on the scanned surface are affected by the arrangement errors of the anamorphic lenses, and a method for solving these problems has not been provided yet.